U.S. Senator John McCain, who recently returned to Washington after treatment for brain cancer, said on Sunday the disease is vicious but that the treatment is going well and he has more energy than before.

"This is a very vicious form of cancer that I'm facing," McCain, a Republican who ran unsuccessfully for president in 2008, told CNN's "State of the Union" program. The 80-year-old Arizona senator was found to have an aggressive form of brain tumor, glioblastoma, after surgery in July for a blood clot above his left eye.

"All the results so far are excellent," McCain said of his treatment, adding that the prognosis was "pretty good." "I'm facing a challenge, but I’ve faced other challenges, and I’m very confident about getting through this one as well," the senator said. "I've had no side effects, no nothing except frankly an increased level of energy."

McCain recently completed his first round of chemotherapy and radiation, and he said that he would have an MRI on Monday, an imaging technique used to evaluate medical conditions.

McCain, who chairs the Senate Armed Services Committee, is also going to oversee work on a defense policy bill on the Senate floor next week.

"Every life has to end one way or another," he told CNN, adding that he was very happy with his life, including the 2008 campaign that he lost to former President Barack Obama.

"I am able to celebrate a wonderful life and I will be grateful for additional time that I have," McCain said, adding: "Remember, I’m the guy who stood fifth from the bottom of his class at the Naval Academy."

McCain was re-elected to a sixth Senate term in November. In the past, he has been treated for melanoma, a form of skin cancer.

He is the son and grandson of Navy admirals, and a U.S. Navy pilot whose plane was shot down over Vietnam in 1967. He spent more than five years as a prisoner of war in North Vietnam and was tortured repeatedly.

McCain said Sunday he hoped that people would remember him as someone who had served his country. "Made a lot of mistakes, made a lot of errors, but served his country, and I hope we could add, honorably."